POWER July 2022 - 40

MARNIE SURFACEBLOW
Beware of Hissing Feedwater
Heaters That Can Bite Your
Bottom Line
Small problems can lead to large impacts on a long-term basis. Heat rate surveys,
improved instrumentation, and utilizing real-time monitoring and diagnostics can
help recover plant efficiency and save money, with payback periods sometimes as
short as weeks.
Una Nowling, PE
M
uted orange light filtered through
slowly drifting clouds of wet
steam, illuminating two figures
walking through the power plant. Marnie
Surfaceblow, vice president and lead
field engineer of Surfaceblow & Associates
International, held out a protective
arm at the edge of a large puddle of water
to stop her lead field engineer and assistant
Maya Sharma. As Maya noted the
leak location on her tablet, Marnie shone
her flashlight across the water. " Grandpa
Marmaduke always said: 'Don't walk
through water on the floor, unless you're
certain it's free of hazards-like an electrical
charge, an ankle-spraining hole, or
Massasauga rattlesnakes.' "
" Ma'am, how many, um, snakes have
you encountered at power plants? "
Detouring around the puddle, Marnie
mused, " More than I care to mention.
Only bitten once, but that was the time
I brought home that baby copperhead
when I was eight years old. I named her
Penny, because, you know, copper. Don't
let anyone tell you that small things can't
cause a lot of trouble-that's one hospital
trip I'll never forget. "
Maya suppressed a shudder, noting
the film noir lighting made every cable
and hose look decidedly snake-like. " Interesting
coincidence, ma'am, since copper
is one of the reasons we are here. "
Marnie continued, " Similarly, small
condenser and feedwater heater issues
can lead to big heat rate problems-
like those condenser air leaks we fixed.
That'll reduce copper loss from the water
tubes and deposits on the turbine
blades, and improve their heat rate by
about 50 Btu/kWh. And thanks to your
noting those problems with their feedwater
heaters by analyzing their DCS
data, we have more work to do. "
" They did not notice it without an on40
line
system alerting them, but I noticed
the drain cooler approach [DCA] of the
#1 feedwater heater increased suddenly
after a trip two months ago and
has remained high, " Maya recalled. " Of
more concern was that the DCA for their
highest-pressure feedwater heater has
increased steadily for months. However,
examining their inspection reports, there
was no indication of extraction line, heat
transfer tube, or partition plate damage
to cause this. "
" Feedwater heaters seem simple, "
said Marnie, " but they require detailed
design, careful maintenance, and consistent
monitoring to maximize their efficiency.
So, your investigation created a
nice little scope adder. Hey, an adder is a
type of snake, interesting coincidence! "
Maya, still imagining snakes in every
air hose and flexible wire conduit, said a
quick prayer, set her teeth, and followed
her boss deeper into the plant.
Unstable Operation Has
Consequences
" This plant was designed for 18 cold
starts per year. Last year, it did 50, and
some of those were after trips from unstable
operation at loads much lower
than design. That's a serious problem,
because a unit trip risks damaging plant
equipment. Case in point, tell me my
young apprentice, what happens to feedwater
heaters during a trip? "
Maya thought for a moment and replied,
" It is a highly unstable situation.
Extraction steam and feedwater flows
can change suddenly. The water level can
rise and the emergency drain valves may
open to protect the turbine from backflow
damage. "
" Very good. Under those conditions,
what happens to the emergency drain
valves? " Marnie asked.
" Because they drain to the condenswww.powermag.com
er,
there is little backpressure, and the
higher-pressure saturated water in the
feedwater heater flashes to steam, accelerating
past the valve, causing potential
erosion and thermal shock. Both
can damage the valve stem and seat,
and the valve may fail to close and leak
directly to the condenser, causing lost
heat rate. "
" Continuing to damage the valve,
steadily increasing the leakage, " Marnie
added, as they arrived at feedwater
heater #1.
An Open Door Is an Invitation
" The DCS indicates all emergency drain
valves are fully closed, ma'am, " said
Maya. Marnie turned and faced Maya,
with her flashlight under her chin. " It
may be telling the truth, or it may be
turning the control room into a den of
lies! " adding an evil laugh for effect.
Maya stared, nonplussed.
" Moving on! " Marnie continued.
" Examine this valve. The position indicator
says it's closed, but see here? " She
pointed with her pen. " See that cleaner
region a few millimeters wide on the
valve stem? It tells me this valve isn't
normally in this position. That could be
just due to normal wear, or seat adjustment.
What would you say? "
Maya examined the valve, noting the
position indicator and seeing there was
more valve stem showing than should
have been. " I agree. This emergency
drain valve is indeed stuck open-maybe
5% or more. And as my auntie Natasha
used to say, 'An open door is an invitation
to things both good and bad.' In this
case, an invitation to heat rate loss. "
Marnie nodded agreement. " Now, given
this is the lowest-pressure heater, the
loss is likely small, maybe 1-3 Btu/kWh?
Still, it's definitely something they should
fix during their next scheduled outage. "
POWER | July 2022
http://www.powermag.com

POWER July 2022

Table of Contents for the Digital Edition of POWER July 2022

POWER July 2022 - Cover1
POWER July 2022 - Cover2
POWER July 2022 - 1
POWER July 2022 - 2
POWER July 2022 - 3
POWER July 2022 - 4
POWER July 2022 - 5
POWER July 2022 - 6
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POWER July 2022 - Cover3
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